OG详解-OG1 数学1 Q10

Choice C is correct. The given system of linear equations can be solved by the substitution method. Substituting -3x  for y from the first equation in the given system into the second equation yields 4x +（-3x) =15 , or x =15.
Choice A is incorrect and may result from conceptual or calculation errors. 
Choice B is incorrect and may result from conceptual or calculation errors. 
Choice D is incorrect. This is the absolute value of y , not the value of x .